Ambassador, British Airways; FlyerTalk Posting Legend
Join Date: Apr 2012
Location: Leeds, UK
Programs: BA GGL/CCR, GfL, HH Diamond
Posts: 43,021
It’s entirely possible to do and, unlike T5, landing yourself in T3 and going out via immigration is pretty easy.
EDIT: I would suggest you have another look since I think a one way fare in ET for £120 is actually quite expensive. I would expect you should be able to find something around half that price at least.